Method for making powder metallurgy shapes

ABSTRACT

PRODUCTION OF POWDER METALLURGY SHAPES BY INTRODUCING A CHARGE OF POWDERED METAL TO BE COMPACTED TO A MOLD HAVING A CONFIGURATION CORRESPONDING GENERALLY TO THE DESIRED CONFIGURATION OF THE FINAL ARTICLE, PLACING THE MOLD IN A CONTAINER HAVING A SECONDARY PRESSURE MEDIA THEREIN AND SURROUNDING THE MOLD, HEATING THE ENTIRE ASSEMBLY TO AN ELEVATED TEMPERATURE FOR COMPACTING AND ISOTATICALLY COMPACTING THE POWDER BY THE APPLICATION OF FLUID PRESSURE WHITLE AT ELEVATE TEMPERAUTE.

Oct. 24, 1972 v. K. cHANDHoK METHOD FOR MAKING POWDER METALLURGY SHAPES Filed March l, 1971 //V VEN TOR V/JAY K. CHANHOK Allorney United States Patent O 3,700,435 METHOD FOR MAKING POWDER METALLURGY SHAPES Vijay Kumar Chaudhok, Pittsburgh, Pa., assigner to Crucible Inc., Pittsburgh, Pa. Filed Mar. 1, 1971, Ser. No. 120,013

Elnt. Cl. B221) l/ U.S. Cl. 75-214 14 Claims ABSTRACT OF THE DISCLOSURE Production of powder metallurgy shapes by introducing a charge of powdered metal to be compacted to a mold having a configuration corresponding generally to the desired configuration of the final article, placing the mold in a container having a secondary pressure media therein and surrounding the mold, heating the entire assembly to an elevated temperature for compacting and isostatically compacting the powder by the application of fluid pressure while at elevated temperature.

It is known in powder metallurgy practice to take a charge of powdered metal, place the same in an evacuated container and after heating the charge to an elevated ternperature place the same in an autoclave wherein the powdered metal is compacted to a density approaching 100% by the application of fluid pressure. By isostatically compacting by the application of fluid pressure it is known that a desirable microstructure and improved properties can be obtained.

Specifically in the manufacture of many articles and particularly tool steel articles, by the combination of isostatically pressing and selected compacting temperatures a heretofore unobtainable microstructure characterized by very small, well distributed carbides in a fine-grained matrix may be obtained, as dscusesd in yU.S. Pat. 3,450,- 528, issued June 17, 1969. In view of the improved properties obtainable with this practice it is desirable to produce, particularly in tool steel applications, tool steel articles of intricate configuration, such as hobs or the like, without requiring extensive machining operations. This is generally difficult to achieve without requiring a series of preheating and precompacting steps to form an article of intermediate density, which of course adds considerably to the cost of the overall practice.

It is accordingly the primary object of the present invention to provide a method for producing intricate shapes by a powder metallurgy practice wherein only a single heating and compacting step is required, thus eliminating the need for any preheating and precompacting.

This and other objects of the invention, as well as a complete understanding thereof, may be obtained from the following description and drawing, the single figure of which is a sectional view through an assembly prepared for compacting in accordance with the present invention.

Broadly, in accordance with the practice of the present invention, a charge of powdered metal to be compacted is introduced to a mold corresponding generally to the configuration desired in the article. The mold is then placed in a container having a secondary pressure media therein, which preferably completely surrounds the mold. This assembly is then heated to an elevated temperature suitable for compacting, which temperature will depend generally upon the composition of the powdered metal charge to be compacted. Finally, the assembly is placed 3,700,435 Patented Oct. 24, 1972 in an autoclave for compacting of the powder by the application of fluid pressure while at elevated temperature. Preferably, the assembly is placed in a furnace for heating and upon reaching compacting temperature is transferred to an autoclave for compacting. 'If desired, however, auxiliary heat may be supplied while the article is in the autoclave. Also the entire heating step could be conducted in the autoclave; however, this practice is not preferred because of the long times required for heating, which may be on the order of two to three hours, during which period the autoclave is not available for compacting. To achieve the desired high quality material in the final compact the container interior is evacuated to remove gaseous impurities plior to compacting. Also, after evacuation, the container is preferably filled with an inert, low molecular weight gas such as helium or hydrogen. The inert gas preferably has a molecular weight of under 28 and provides for more efficient heat conduction within the mold. This serves to speed up heating to the desired elevated compacting temperature and accordingly renders the practice more economical. The gas should be one which is substantially nonreactive with the powdered metal being compacted and for this reason hydrogen and helium, by reason of their nonoxidizing character and most irnportantly, because of their low molecular weight and hence high rate of conductive heat transfer are preferred. After heating to the desired temperature, the gas is removed and the assembly again evacuated prior to compacting, The mold, for purposes of obtaining a high quality final product, should be constructed of a material that is inert with respect to the metal of the powder being compacted. For this purpose silica, zirconia, alumina and mixtures thereof as well as compatible metals and alloys are preferred for use as the mold material. In the preferred practice of the invention and for the purpose of avoiding the use of auxiliary heating in the autoclave, and thus the provision of heating elements for this purpose therein, it is preferred to heat the powered charge in a furnace positioned adjacent the autoclave to a temperature in excess of that required during subsequent compacting. The charge is then transferred to the autoclave and compacted prior to cooling below the required, selected compacting temperature.

During compacting densities approaching are achieved generally by the application of pressure within the range of 10,000 to 30,000 p.s.i. For material such as tool steel compacting temperatures on the order of about 1800 to about 2300 F. are typically used. Although the practice is suitable for use with powders of various sizes, it is generally preferred that the size of the particles be not larger than about minus 30 mesh.

After compacting the mold is removed from the container and secondary pressure media and then is removed from the compact by sand blasting or pickling.

With reference to the drawing, there is shown an assembly ready for compacting and designated generally as 10. The assembly consists of a mold 12, which may be of silica, zirconia, alumina or mixtures thereof. The mold 12 is filled with a powder charge 14, of the metal or alloy desired in the final product. During filling of the mold it is customary to agitate the same so as to insure complete filling with the powder charge. The mold is placed within a container 20, which may be constructed of mild, carbon steel. 'Ihe container 20 has a stem 21. The container 20 is -lled with a secondary pressure media 22 which may be silica and alumina in finely divided form. The secondary pressure media 22 preferably completely fills the container and surrounds the mold 12.

With the assembly constructed as shown in the drawing the interior is outgassed. This requires the connection of the chamber interior via stem 21 to a suitable vacuum pump for removal of gaseous reaction products produced therein during heating. For this purpose heating to a relatively low temperature of about 400 to 500 F. is generally satisfactory. lf outgassing is not provided for, the resulting compact may be characterized by the presence of detrimental oxides and other impurities which may adversely affect both bonding of the powder and the quality of the final product. After outgassing, which results in evacuation of the chamber interior and mold interior, the chamber and mold interiors are preferably filled with an inert gas of low molecular weight such as helium or hydrogen. This reduces the time required for subsequent heating to compacting temperature. After heating to compacting temperature, the gas is removed from the interior, as by pumping, and the interior is again evacuated. The stem 21 is then closed to seal the interior against the atmosphere and then the entire assembly as shown in the drawing is transferred to an autoclave for isostatic compacting by the application of uid pressure. This apparatus may be that shown and described in U.S. Pat. 3,543,345, issued Dec. 1, 1970. In the autoclave pressures on the order of 10,000 to 30,000 p.s.i. are generally used and by the combination of elevated temperature and uniform pressure application of this magnitude the powdered metal is compacted to a density of about 100% and conforms substantially to the configuration of the mold. In this regard, it is to be understood that the mold 12 as shown in the drawing may be of various configurations depending upon the product to be produced.

Upon removal of the mold from the compact a product of the desired configuration is achieved and generally very little, if any, machining is required. By the practice above described in accordance with the present invention it is not necessary to preform the powdered metal to a compact of intermediate density prior to final heating and compacting to final densities. Consequently, with the practice of the invention a one-step operation is possible. This is in contrast with prior practices wherein it was necessary to form an intermediate compact either by cold pressing and using a binder in admixture with the powdered metal to achieve the required strength and integrity or alternately heating the metal to an elevated temperature at which the combination with compacting pressure produced an intermediate compact having the strength required for subsequent handling prior to final heating and compacting.

As a specific example of the practice of the invention, a charge of MZS tool-steel powder of about minus 100 mesh was placed within a cylindrical shell-type mold constructed from alumina. The mold was agitated for a short period to insure filling of the container and then it was placed within a mild steel container which was filled with silica particles to surround the mold and act as a secondary pressure media during subsequent compacting. The assembly was then heated to achieve a powder charge temperature of about 100 to 200 F. at which time pumping was begun and the container and mold interiors were evacuated. Thereafter, the interior was filled with helium. The assembly was then heated to achieve powder charge temperature of about 2l75 F., helium was removed and the container transferred to an autoclave and compacted by the application of fluid pressure on the order of 16,000 p.s.i. to a final density approaching 100%. The final product was of excellent integrity and conformed to the cylindrical shape of the mold.

It is understood that the term powder metal as used herein means both metals and alloys thereof in particle form.

I claim:

1. A method for producing a powder metallurgy article, comprising forming an assembly by introducing powder metal to a mold corresponding generally to the configuration of said article and placing said mold in a container sealed against the atmosphere and having a secondary pressure media therein, thereafter heating said assembly to an elevated temperature for compacting and isostatically compacting said powder by the application of fluid pressure to said assembly while at an elevated temperature.

2. The method of claim 1 wherein said heating is conducted in a furnace and said assembly is transferred from said furnace to an autoclave for said compacting after heating to said elevated compacting temperature.

3. The method of claim 1 wherein said mold with said powder metal therein is evacuated prior to said compacting.

4. The method of claim 1 wherein said mold with said powder metal therein is sequentially evacuated and filled with an inert gas prior to said heating to compacting temperature.

5. The method of claim 1 wherein said mold is constructed from a material that is inert with respect to said powder under conditions of heating and compacting.

6. The method of claim 5 wherein said mold is constructed from a material selected from the group consisting of zirconia, alumina and mixtures thereof.

7. A method for producing a powder metallurgy artcle, comprising forming an assembly by introducing powder metal to a mold corresponding generally to the configuration of said article, said mold being constructed from a material that is inert with respect to said powder metal under conditions of heating and compacting and placing said mold in a container sealed against the atmosphere and having a secondary pressure media therein and surrounding said mold, thereafter evacuating said mold interior and heating said assembly to an elevated temperature, and isostatically compacting said powder by the application of fluid pressure to said assembly while at elevated temperature.

8. The method of claim 7 wherein said assembly is heated in a furnace to said elevated temperature and thereafter transferred to an autoclave for said isostatic compacting.

9. The method of claim 8 wherein said powder metal is heated to a temperature above the temperature at which it is compacted prior to introduction of said assembly to said autoclave for said compacting.

10. The method of claim 9 wherein said powder metal is subjected to additional heating while within said autoclave and prior to compacting.

11. The method of claim 7 wherein said mold with said powder metal therein is filled with an inert gas intermediate said evacuation and heating of said powder metal.

12. The method of claim 7 wherein said fluid pressure media is an inert gas selected from the group consisting of helium, argon, hydrogen and nitrogen.

13. A method for producing a powder metallurgy article, comprising introducing powder metal to a mold corresponding generally to the configuration of said article, said mold being constructed from a material that is inert with respect to said powder metal under conditions of subsequent heating and compacting, placing said mold in a container having a solid-form secondary pressure media therein and surrounding said mold, heating said mold and container, evacuating the interiors of said mold and container, filling said interior of said mold and container with an inert gas, heating said mold and container in a furnace to increase the temperature of said powder metal above a selected compacting temperature, removing said inert gas, sealing the container against the atmosphere, transferring said container and mold therein to a fluid pressure vessel and compacting said powder metal by the application of iiuid pressure while at a temperature above said selected compacting temperature.

5 14. The method of claim 13 wherein said compacting is achieved in the absence of further heating in said uid pressure vessel.

References Cited UNITED STATES PATENTS 3,599,281 8/1971 Boyer 75-226 X 2,725,288 11/ 1955 Dodds et al 75-226 3,419,935 1/1969 Pfeiler et al. 75226 X 6 3,571,850 3/1971 Pohto et al 75--226 X 3,383,208 5/1968 Corral 75-226 X CARL D. QUARFORTH, Primary Examiner 5 R. E. SCHAFER, Assistant Examiner U.S. C1. X.R. 

